1. Field of the Invention
The present invention relates to a laser beam printer, digital copier, laser facsimile apparatus or similar image forming apparatus and a multiple-beam scanning device included in an image forming apparatus and capable of adjusting a beam spot position in the subscanning direction at an end image height.
2. Description of the Background Art
To meet the increasing demand for a high density, high speed image forming apparatus, various schemes have heretofore been proposed in relation to a scanning device or scanning means included in an image forming apparatus. For example, the conventional schemes include one that increases the rotation speed of a polygonal mirror or similar deflector included in the scanning device and one that increases the number of light sources.
Increasing the rotation speed of a deflector, however, brings about noise, vibration, temperature elevation and other problems. These problems cannot be solved without increasing cost. The rotation speed cannot therefore be increased above a certain limit.
On the other hand, to increase the number of light sources, use is made of, e.g., means for combining light beams issuing from a plurality of light sources or a laser diode array including a plurality of light sources or light emitting portions. When high density and high speed are not achievable by increasing the rotation speed of the deflector, increasing the number of light sources, i.e., a so-called multiple-beam scanning device is effective.
The prerequisite with a multiple-beam scanning device is that the distance between nearby beam spot positions in the subscanning direction in an image plane, i.e., a so-called beam pitch be maintained constant. For example, when scanning line density is 600 dpi (dots per inch), the beam pitch must be maintained at 42.3 μm. Such a beam pitch must be insured without regard to the environment in which the scanning device is operated.
In practice, however, the refractive index of an optical device, disposed in an optical scanning device, is not uniform. Moreover, it is impossible to mount the optical device with accuracy precisely equal to designed accuracy due to limited machining accuracy. As a result, light beams from light sources do not accurately pass the focus of the optical device, preventing a desired beam pitch from being established.
In fact, although any desired beam pitch can be implemented by adjustment on, e.g., a production line, the adjustment is effected only at the center of an image or center image height. Consequently, it is likely that the beam pitch at the end image height differs from the beam pitch at the center image height. Particularly, in a color image forming apparatus, a difference between beam pitches at different image heights, i.e., a beam pitch deviation noticeably appears in an image, as known in the art. When a plurality of color prints are output, the tone of color conspicuously varies from one image to another image.
Further, an optical device is generally implemented as an elongate optical device because it scans the surface of a photoconductive element or subject surface. An elongate optical device is extremely susceptible to the variation of refractive index ascribable to the temperature deviation or heat distribution of the environment around the optical device. Consequently, even if adjustment is so made as to establish a preselected beam pitch at the center image height, the preselected beam pitch is unable to be established at an end image height, also resulting in defective images.
The beam pitch deviation between image heights, of course, occurs when parallelism between nearby beams on an image plane is distorted, i.e., when the bends or the inclinations of nearby scanning lines formed by beams differ from each other. However, it is difficult to correct the bend or the inclination of the individual scanning line.
To correct the inclinations of scanning lines, Japanese Patent Laid-Open Publication No. 10-175324, for example, teaches adjusting means configured to angularly move a return mirror in the lengthwise direction. However, providing the return mirror with eccentricity translates into varying optical length between image heights, so that the focus of a beam spot differs from one image height to another image height. This brings about a magnification error in the main scanning direction and the degradation of the beam spot diameter and prevents, in a color image forming apparatus, toner images of different colors from being superposed in accurate register.
Japanese Patent Laid-Open Publication No. 5-24108, for example, proposes to establish a preselected beam pitch by controlling the angle of a return mirror in such a manner as to provide it with β eccentricity. This method, however, cannot correct the beam pitch deviation between different image heights.
Japanese Patent Laid-Open Publication No. 7-113973, for example, teaches means for moving an optical device in the direction of optical axis. A problem with this scheme is that because the optical device is supported by a lens holder, a large number of parts are necessary while the errors of individual parts accumulate during assembly. Consequently, adequate optical characteristics are not attainable without increasing the accuracy of the individual parts, resulting in an increase in cost.
Moreover, none of the conventional schemes described above includes correcting means for a multiple-beam configuration.
Technologies relating to the present invention are also disclosed in Japanese Patent Laid-Open Publication Nos. 5-241087 and 9-90187.